I. Field of the Invention
This invention relates generally to cardiac pacing apparatus, and more particularly to a variable rate pacemaker which incorporates a sensor for monitoring hemodynamic performance and for establishing the pacing rate as determined by the hemodynamic performance of the heart.
II. Discussion of the Prior Art
There exists a class of cardiac pacemakers known as variable rate or rate adaptive pacemakers which include a first sensor for determining metabolic demand and means for adjusting the pacing rate or escape interval of the stimulating pulse generator based upon that metabolic demand. For example, the Anderson et al. U.S. Pat. No. 4,428,378 discloses a rate adaptive pacer having an activity sensor which produces an electrical signal related to the level of motion or activity of the patient in whom the pacemaker is implanted. The activity signal is signal-processed and applied to a voltage-controlled oscillator for adjusting the rate at which electrical stimulating pulses are produced. The pacemaker has a predetermined, programmed lower stimulating pulse rate, and sensed activity results in a pulse rate increase from that lower rate threshold up to a predetermined, programmed maximum rate.
There are a variety of other rate adaptive pacers in which parameters other than motion or activity are sensed. For example, the Begemann et al. U.S. Pat. No. 4,972,834 describes such a pacemaker in which the QT interval of the ECG cycle is measured and because that interval varies with physiologic demand, it is used to adjust the pacing rate or escape interval of a stimulating pulse generator such that the rate varies with physiologic demand. Again, a programmed lower rate limit and upper rate limit is provided to assure that the patient is not paced at a rate which may prove harmful. The upper rate limit is, however, established by the physician and programmed into the pacemaker as a fixed limit.
Other parameters that have been sensed and used to control pacing rate have included right ventricular systolic pressure (Koning et al. U.S. Pat. No. 4,566,456); blood pH (Alcidi U.S. Pat. No. 4,009,721); blood oxygen saturation (Wirtzfeld et al. U.S. Pat. No. 4,202,339); respiration rate (Krasner U.S. Pat. No. 3,593,718); partial pressure of carbon dioxide in the blood, pCO.sub.z, (Koning et al. U.S. Pat. No. 4,716,887); blood temperature (Cook et al. U.S. Pat. No. 4,436,092); and pre-ejection period (Chirife U.S. Pat. No. 4,719,921).
The foregoing list of prior art patents relating to rate adaptive pacemakers is merely illustrative in that other patents have been granted. Hence, the above listing is not intended to be exhaustive.
Typical adaptive rate pacemakers, such as those described in the aforementioned patents, are primarily operated so as to increase the pacing rate in responses to a change in the parameter being sensed. However, if the pacing rate is allowed to increase inappropriately in a pathological heart, inefficiencies may result in the heart's pumping function, resulting in hemodynamic instability. That is to say, the patient may become uncomfortable, experience shortness of breath and cease exercising. Thus, it is desirable to establish a match between the pacing rate and the optimal pump function in that exercise tolerance would improve at both maximal and submaximal levels of effort.
Prior art adaptive rate pacemakers typically involve the simple conversion of a sensor value to a paced rate. The sensor index may be of various technologies, all as pointed out above. These technologies include piezoelectric motion or activity sensing, accelerometer-based activity, transthoracic impedance measures, right ventricular impedance measures and others. A study of prior art cardiac pacers also reveals that considerable attention has been paid to the establishment of rate response curves used to convert a sensor signal to a pacing rate control signal between the lower rate limits and maximum sensor rate. A combination of sensors may be used simultaneously or in concert for developing the desired rate response curve. For example, activity may be used to initiate rate response and a proportional control may be provided by sensing temperature or minute ventilation indices once the pacing rate has exceeded some predetermined value above the base or at-rest value. In accordance with the present invention, a pacemaker design and an algorithm is provided whereby the pacemaker is prevented from inducing hemodynamic failure due to an inappropriate pacing rate, which might otherwise result from conventional, prior art rate response algorithms. The apparatus and method of the present invention constitutes a departure from the prior art in that it offers a hemodynamic upper rate limit that minimizes the occurrence of hemodynamic instability arising from inappropriate pacing therapy. While conventional, prior art adaptive rate pacing devices have classicly used predetermined lower and maximum rate limits which are prescribed by the implanting physician, in accordance with the present invention a further sensor and/or algorithm is provided for quantifying hemodynamic performance and establishing an upper rate when it is determined that a further increase in pacing rate will result in a worsening of hemodynamic performance.
By adding a hemodynamic sensor to a rate adaptive pacer where the hemodynamic sensor quantifies the hemodynamic stability of the heart in response to pacing stimulation, the consequences of an inappropriately high pacing rate can be obviated. If the heart is paced too fast, it may not be able to adequately improve cardiac output. In a normal heart, at excessively high rates, the cardiac output decreases as the rate increases. This induced hemodynamic instability usually does not occur until there are higher paced rates, i.e., in excess of 150-200 bpm. However, in the pathologic heart, less cardiac reserve is present. Hence, if paced at too high a rate, severe shortness of breath and hemodynamic instability may result. While in prior art systems provision has been made for programming in an upper rate limit to control this problem, the present invention provides a way of optimally governing the upper rate limit or maximum sensor rate, based upon monitoring of the resulting hemodynamic performance of the heart.